The band structure for single crystal silicon exhibits a conduction band minimum which does not have the same crystal momentum as the valence band maximum yielding an indirect gap, see S. M. Sze, Physics of Semiconductor Devices, 2nd ed., John Wiley, New York, 1981, p. 13. Therefore, in silicon, radiative recombination can only take place with the assistance of a phonon source of light, unlike group III-V semiconductors which have a direct gap at the center of the Brillouin zone. The discovery of photoluminescence in porous silicon has therefore generated a now optoelectronic material for study, note, for example, the articles by L. T. Canham, Appl. Phys. Lett., vol. 57, 1990, p. 1046; by V. Lehmann and U. Gosele, Appl. Phys. Lett., vol. 58, 1990, p. 856: by A. Bsiesy et al., Surf. Sci., vol. 254, 1991, p. 195; by A. Richter et al., IEEE Electron Device Letters, vol. 12, 1991, p. 691; by A. G. Cullis and L. T. Canham, Nature, vol. 353, 1991, p. 335. by V. V. Doan and M. J. Sailor, Science, vol. 265, 1992, p. 1791; by J. C. Campbell et al., Appl. Phys. Lett, vol. 60, 1992, p. 889: by V. V. Doan and M. J. Sailor, Appl. Phys. Lett., vol. 60, 1992, p. 619; by A. Halimaoui et al., Appl. Phys. Lett., vol. 59, 1991, p. 304; and by N. Koshida and H. Koyama, Appl. Phys. Lett., vol. 60, 1992, p. 347.
FIG. 1 schematically shows a cross-section of a porous silicon layer PS formed on a bulk silicon substrate BSS. The porous layer is typically formed using electrochemical etching as described in the above references to a depth of about 1 to 30 microns into the bulk silicon substrate, which is nominally 500 microns thick. The typical emission spectrum of electrochemically etched porous silicon is in the red, orange and yellow region (nominally 500 to 750 nm) although green and blue emission has also been demonstrated. Blue shift of the peak emission wavelength has been shown by increased oxidation and etching of the porous silicon, see S. Shih et al., Appl. Phys. Lett., vol. 60, 1992, p. 1004.
At this time the light emitting mechanism is not fully understood, with three competing theories existing: quantum size effects, as referred to in Canham, 1990 and Shih et al., 1992 cited above, amorphous silicon radiative emission, as referenced in R. P. Vasquez et al., Appl. Phys. Lett, vol. 60, 1992, p. 1004, and surface passivation species allowing molecular radiative emission, see X. L. Zheng et al., Appl. Phys. Lett., vol. 60, 1992, p. 986; and M. S. Brandt et al., Solid State Comun, vol. 81, 1992, p. 307. This scientific controversy, however, has not hindered the ability to fabricate porous silicon layers and useful light emitting devices using this technology in bulk silicon.
Thus, in accordance with this inventive concept a need has become apparent for a light emitting (photonic) silicon an a transparent substrate and its method of fabrication which provides improved additional capabilities and also offers compatibility with advanced microelectronic technology.